We propose to test the Female Constraint Hypothesis (Gowaty 1995b) in eastern bluebirds, Sialia sialis, American robins, Turdus migratorious, and European starlings, Sturnus vulgaris. The Female Constraint Hypothesis says that females' vulnerabilities to social coercion of their mating choices (choice of gametic partners) by their territorial male partners is a function of females' intrinsic abilities and the richness of the environments that females find themselves in. The Female Constraint Hypothesis makes four crucial predictions: 1. extra pair paternity will be higher in tie broods of "more able females" (meaning females with excellent foraging skills and/or those in prey-rich environments) than "less able females"; 2. among females with extra pair paternity in their broods, genetic monogamy is more frequent among "more able" than "less able" females; 3. variance in reproductive success of males mated to "more able" females is greater than variance in reproductive success of males mated to "less able" females. 4. Males mated to "more able" females achieve more extra pair paternity than males mated to "less able" females. We will test each of these within-species predictions using observations and experiments of wild-living populations of each of three species. We will also test crucial interspecific predictions via comparisons of eastern bluebirds and American robins, two thrush species in the same subfamily, Turdinae. These comparisons will allow us to make inferences about the effects of ecological variation on differences between the species, because the confounding effects of shared phylogeny will be held constant. We will also test other interspecific predictions via comparisons of eastern bluebirds and European starlings, two cavity nesting species from different families within the Passeriformes. These comparisons will allow us to make inferences about the effects of ecology on similarities between these species, because the confounding effect of shared phylogeny will be absent. We will experimentally test the FCH by experimental manipulation of prey and nesting cavities used by eastern bluebirds. We will use up to ten highly variable species specific DNA microsatellite primers in "DNA typing" useful for genetic parentage testing; we will use field observations of naturally occurring behavior, and experimental manipulations of wild-living individuals in our tests of these ideas.